The Effect Of Topically Applying Five Thermoreceptor-stimulating Chemicals On Local Thermal Sensation And Pain Thresholds

PURPOSE: To determine the effect of menthol (MEN), capsaicin (CAP), camphor (CAM) eugenol (EUG), and cinnamaldehyde (CIN) on local thermal sensation and pain thresholds of the forearm, compared to a control (Vaseline; CON) trial. METHODS: On six separate occasions, 8 participants (4 male, 4 female; 27 ± 5 y; 171 ± 9 cm; 74.5 ± 10.3 kg) sat passively in a thermoneutral environment (21.5 ± 0.5 °C and 50 ± 3% relative humidity), while temperature stimuli were applied to the ventral surface of the forearm using a peltier device coated in one of the six aforementioned chemicals, while thermal sensation was recorded on a 200 mm visual analog scale (0 mm = extremely cold, 200 mm = extremely hot). The temperature stimuli ranged from 10 to 46 °C in two-degree increments (19 stimuli total), delivered in a randomized order, and were batched as COLD (10-16 °C), COOL (18-24 °C). NEUTRAL (26-32 °C), WARM (24-40 °C) and HOT (42-46 °C) for analysis. Subsequently, the detection thresholds for warm and cool sensation and pain thresholds for hot and cold sensations were determined by temperature-ramp protocols. RESULTS: All five chemicals were compared to the CON trial (COLD: 55 ± 19 mm; COOL: 78 ± 9 mm; NEUTRAL: 100 ± 4 mm; WARM: 122 ± 11 mm; HOT: 148 ± 7 mm). MEN caused cooler sensations in the COLD (36 ± 20 mm, P = 0.02) and COOL (55 ± 19 mm, P = 0.01) temperature ranges. CIN caused warmer sensations in the COOL (87 ± 8 mm, P = 0.05) and HOT (163 ± 13 mm, P = 0.02) temperature ranges. CAP caused warmer sensations in the WARM (147 ± 24 mm, P = 0.05) and HOT (174 ± 18 mm, P = 0.01) temperature ranges. No differences were observed with EUG and CAM at any temperature range (P > 0.24). In the CON trial, detection of cool sensation occurred at 24.2 ± 1.2 °C, cold pain at 2.2 ± 2.9 °C, warm sensation at 39.2 ± 2.3 °C, and hot pain at 48.2 ± 1.9 °C. With CAP, detection of warm sensation (36.0 ± 2.8 °C, P = 0.002) and hot pain at (44.5 ± 5.0 °C, P = 0.005) occurred at lower temperatures. With CIN, detection of hot pain (47.6 ± 2.3 °C, P = 0.05) occurred at a lower temperature. No difference to CON were observed in the EUG and CAM trials (P > 0.19). CONCLUSIONS: Menthol lowered thermal sensation in the cooler temperature ranges and capsaicin increased thermal sensation in the warmer temperature ranges, demonstrating chemically mediated thermal perception is range dependent and does not occur across all temperatures.